Nitride semiconductor light-emitting device and optical apparatus including the same

ABSTRACT

A nitride semiconductor light emitting device includes an emission layer ( 106 ) having a multiple quantum well structure where a plurality of quantum well layers and a plurality of barrier layers are alternately stacked. The quantum well layer is formed of XN 1-x-y As x P y Sb z  (0≦x≦0.15, 0≦y≦0.2, 0≦z≦0.05, x+y+z&gt;0) where X represents one or more kinds of group III elements. The barrier layer is formed of a nitride semiconductor layer containing at least Al.

TECHNICAL FIELD

[0001] The present invention relates to a nitride semiconductor lightemitting device with high luminous efficiency and an optical apparatususing the same.

BACKGROUND ART

[0002] Conventionally, nitride semiconductor has been used or studiedfor a light emitting device or a high power semiconductor device. In thecase of a nitride semiconductor light emitting device, a light emittingdevice in a wide color range from blue to orange can be fabricated.Recently, by utilizing the characteristics of the nitride semiconductorlight emitting device, a blue or green light emitting diode or a bluishviolet semiconductor laser has been developed. Japanese PatentLaying-Open No. 10-270804 has reported a light emitting device includingan emission layer formed of GaNAs well layers/GaN barrier layers.

[0003] In the GaNAs/GaN emission layer described in Japanese PatentLaying-Open No. 10-270804, however, crystal phase separation of ahexagonal system having a high N concentration and a cubic system (zincblende structure) having a low N concentration is liable to occur, andthus a light emitting device having a high luminous efficiency is hardlyobtained.

DISCLOSURE OF THE INVENTION

[0004] A main object of the present invention is therefore to enhancethe luminous efficiency in a nitride semiconductor light emitting deviceincluding an emission layer having a quantum well structure formed ofnitride semiconductor by improving crystallinity of the emission layerand suppressing crystal phase separation.

[0005] According to the present invention, a nitride semiconductor lightemitting device includes an emission layer having a multiple quantumwell structure where a plurality of quantum well layers and a pluralityof barrier layers are alternately stacked. The quantum well layer isformed of XN_(1-x-y-z)As_(x)P_(y)Sb_(z) (0≦x≦0.15, 0≦y≦0.2, 0≦z≦0.05.x+y+z>0), where X represents one or more kinds of group III elements.The barrier layer is formed of a nitride semiconductor containing atleast Al.

[0006] In this way, the emission layer for causing effect of emittinglight includes the quantum well layers and the barrier layers, and thequantum well layer has a smaller energy band gap as compared with thebarrier layer.

[0007] It is preferable that the barrier layer further contains In. Itis also preferable that the barrier layer further contains any elementselected from the group consisting of As, P and Sb.

[0008] The nitride semiconductor light emitting device includes asubstrate and the emission layer has a first main surface closer to thesubstrate and a second main surface farther from the substrate. In afirst adjacent semiconductor layer in contact with the first mainsurface of the emission layer and a second adjacent semiconductor layerin contact with the second main surface of the emission layer, at leastthe second adjacent semiconductor layer is preferably formed of anitride semiconductor containing at least Al. One of the quantum welllayer is preferably in direct contact with the first adjacentsemiconductor layer or the second adjacent semiconductor layer.

[0009] Al content in the barrier layer is preferably at least 5×10¹/cm³.In group V elements in the barrier layer, preferably, As content is atmost 7.5%, P content is at most 10% and Sb content is at most 2.5%.

[0010] The emission layer preferably includes at least two to at mostten well layers. The quantum well layer preferably has a thickness of atleast 0.4 nm and at most 20 nm. The barrier layer preferably has athickness of at least 1 nm and at most 20 nm.

[0011] At least one kind of dopant selected from the group consisting ofSi, O, S, C, Ge, Zn, Cd and Mg is preferably added to the well layersand/or the barrier layers. The amount of such added dopant is preferablywithin a range of 1×10¹⁶-1×10²⁰/cm³.

[0012] GaN may preferably be used as a substrate material of the nitridesemiconductor light emitting device.

[0013] The nitride semiconductor light emitting device described abovemay preferably be used in a variety of optical apparatuses such as anoptical information reading apparatus, an optical information writingapparatus, an optical pickup, a laser printer, a projector, a display, awhite light source apparatus and the like.

BRIEF DESCRIPTION OF THE DRAWINGS

[0014]FIG. 1 is a schematic cross sectional view showing a structure ofa nitride semiconductor laser device according to an embodiment of thepresent invention.

[0015]FIG. 2 is a schematic top view illustrating chip division of thelaser device according to the embodiment.

[0016]FIG. 3 is a graph showing relation between the number of welllayers and the threshold current density in the laser devices.

[0017] FIGS. 4(a) and 4(b) are diagrams schematically showing energyband gap structures in the light emitting devices according to theembodiments.

[0018] FIGS. 5(a) and 5(b) are diagrams schematically showing otherexamples of the energy band gaps in the light emitting devices accordingto the embodiments.

[0019]FIG. 6 is a diagram schematically showing the energy band gapstructure in a conventional light emitting device.

[0020]FIG. 7 is a schematic cross sectional view showing a structure ofa laser device using a nitride semiconductor substrate as an embodiment.

[0021]FIG. 8 is a schematic cross sectional view showing a nitridesemiconductor thick-film substrate that may be used in the lightemitting device according to the present invention.

[0022]FIG. 9(a) is a schematic cross sectional view showing an exemplarylight emitting diode device according to the present invention and FIG.9(b) is a schematic top view corresponding to the diode device of FIG.9(a).

[0023]FIG. 10 is a graph showing relation between the number of welllayers and the emission intensity of the light emitting diode devicesaccording to the present invention.

[0024]FIG. 11 is a schematic block diagram showing an optical diskrecording/reproducing apparatus as an exemplary optical apparatus usingthe light emitting device according to the present invention.

BEST MODES FOR CARRYING OUT THE INVENTION

[0025] Various embodiments of the present invention will be described inthe following.

[0026] Generally, GaN, sapphire, 6H-SiC, 4H-SiC, 3C-SiC, Si, Spinel(MgAl₂O₄) or the like is used as a substrate material to grow a nitridesemiconductor crystal layer. Similarly as a GaN substrate, anothersubstrate made of nitride semiconductor can also be used. For example,Al_(x)Ga_(y)In_(z)N (0≦x≦1, 0≦y≦1, 0≦z≦1, x+y+z=1) substrate can beused. In the case of a nitride semiconductor laser, in order to attain aunimodal vertical lateral mode, a layer having a refractive index lowerthan that of a cladding layer needs to be in contact with the outer sideof the cladding layer, and an AlGaN substrate is preferably used.Furthermore, Si, O, Cl, S, C, Ge, Zn, Cd, Mg or Be may be doped in thesubstrate. For an n-type nitride semiconductor substrate, Si, O and Clis particularly preferable among these doping agents.

[0027] While in the following embodiments a sapphire substrate and aC-plane {0001} substrate of nitride semiconductor will mainly bedescribed among the substrates as described above, an A-plane {11-20},an R-plane {1-102} or an M-plane {1-100} may be used other than theC-plane as a surface orientation of the main surface of the substrate.If a substrate has an off-angle within 2° from those surfaceorientations, the semiconductor crystal layer grown thereon has a goodsurface morphology.

[0028] Metal-organic chemical vapor deposition (MOCVD), molecular beamepitaxy (MBE), hydride vapor phase epitaxy (HVPE) and the like aregenerally used to grow the crystal layer. Among those, it is mostgeneral to use GaN or sapphire as a substrate and to use MOCVD to growcrystal, considering the crystallinity and productivity of thefabricated nitride semiconductor layer.

[0029] (Embodiment 1)

[0030] In the following, a nitride semiconductor laser diode deviceaccording to an embodiment 1 of the present invention will be described.

[0031] The nitride semiconductor laser diode device according toembodiment 1 shown in the schematic cross section of FIG. 1 includes aC-plane (0001) sapphire substrate 100, a GaN buffer layer 101, an n-typeGaN contact layer 102, an n-type In_(0.07)Ga_(0.93)N anti-cracking layer103, an n-type Al_(0.1)Ga_(0.9)N cladding layer 104, an n-type GaN lightguide layer 105, an emission layer 106, a p-type Al_(0.2)Ga_(0.8)Nblocking layer 107, a p-type GaN light guide layer 108, a p-typeAl_(0.1)Ga_(0.9) cladding layer 109, a p-type GaN contact layer 110, ann-type electrode 111, a p-type electrode 112 and an SiO₂ dielectric film113.

[0032] In fabrication of the laser diode device of FIG. 1, sapphiresubstrate 100 is first placed in an MOCVD apparatus, and GaN bufferlayer 101 is grown to a thickness of 25 nm at a relatively low substratetemperature of 550° C. using NH₃ (ammonia) as a source for a group Velement N and TMGa (trimethyl gallium) as a source for a group IIIelement Ga. Then, in addition to the sources for N and Ga describedabove, SiH₄ (silane) is also used to grow n-type GaN contact laywer 102(Si impurity concentration: 1×10¹⁸/cm³) to a thickness of 3 μm at atemperature of 1050° C. Thereafter, the substrate temperature isdecreased to about 700° C. to 800° C., and using TMIn (trimethyl indium)as a source for a group III element In, n-type In_(0.07)Ga_(0.93)Nanti-cracking layer 103 is grown to a thickness of 40 nm. The substratetemperature is again increased to 1050° C., and using TMAl (trimethylaluminum) as a source for a group III element Al, n-typeAl_(0.1)Ga_(0.9)N cladding layer 104 (Si impurity concentration:1×10¹⁸/cm³) is grown to a thickness of 0.8 μm, followed by n-type GaNlight guide layer 105 (Si impurity concentration: 1×10¹⁸/cm³) grown to athickness of 0.1 μm.

[0033] Thereafter, the substrate temperature is decreased to 800° C. andemission layer 106 is formed having a multiple quantum well structurealternately stacked with a plurality of Al_(0.02)In_(0.07)Ga_(0.9)Nbarrier layers of 6 nm thickness and a plurality of GaN_(1-x)As_(x)(x=0.02) well layers of 4 nm thickness. In this embodiment, emissionlayer 106 has a multiple quantum well structure starting from thebarrier layer and ending at the barrier layer and includes three quantumwell layers. In growing these barrier layers and well layers, SiH₄ isadded so that both of them have a Si impurity concentration of1×10¹⁸/cm³. It is noted that a growth interruption interval of not lessthan one second and not more than 180 seconds may be inserted betweenthe growth of the barrier layer and the well layer or between the growthof the well layer and the barrier layer. With the growth interruptionintervals, the flatness of the barrier layers and the well layers isimproved and then the emission half-width can be reduced.

[0034] Generally, the content of As, P or Sb in the well layer can beadjusted according to a targeted emission wavelength of a light emittingdevice. In order to obtain an emission wavelength in the vicinity of 410nm of violet, for example, x may be 0.02 in GaN_(1-z)As_(x), y may be0.03 in GaN_(1-x)P_(y) and z may be 0.01 in GaN_(1-z)Sb_(z).Furthermore, in order to obtain an emission wavelength in the vicinityof 470 nm of blue, x may be 0.03 in GaN_(1-x)As, y may be 0.06 inGaN_(1-y)P_(y) and z may be 0.02 in GaN_(1-z)Sb_(z). In addition, inorder to obtain an emission wavelength in the vicinity of 520 nm ofgreen, x may be 0.05 in GaN_(1-x)As_(x), y may be 0.08 in GaN_(1-y)P_(y)and z may be 0.03 in GaN_(1-z)Sb_(z). Still further, in order to obtainan emission wavelength in the vicinity of 650 nm of red, x may be 0.07in GaN_(1-x)As_(x), y may be 0.12 in GaN_(1-y)P_(y) and z may be 0.04 inGaN_(1-z)Sb_(z).

[0035] In a case where InGaNAs-based or InGaNP-based semiconductor isused for a well layer, the numeral values shown in Tables 1 and 2 may beemployed as a value of a content proportion x depending on a contentproportion y of In, in order to obtain the targeted emission wavelength.TABLE 1 In_(y)Ga_(1-y)N_(1-x)As_(x) In(y = 0.01) In(y = 0.02) In(y =0.05) In(y = 0.1) In(y = 0.2) In(y = 0.35) In(y = 0.5) emissionwavelength 400 nm 0.012 0.011 0.008 0.003 410 nm 0.016 0.015 0.011 0.006470 nm 0.034 0.033 0.029 0.024 0.014 0.001 520 nm 0.046 0.045 0.0410.036 0.025 0.012 0.001 650 nm 0.07  0.069 0.065 0.059 0.048 0.034 0.023

[0036] TABLE 2 In_(y)Ga_(1−y)N_(1−x)P_(x) In(y = 0.01) In(y = 0.02) In(y= 0.05) In(y = 0.1) In(y = 0.2) In(y = 0.35) In(y = 0.5) emissionwavelength 400 nm 0.02  0.018 0.013 0.004 410 nm 0.025 0.023 0.018 0.01 470 nm 0.055 0.053 0.047 0.038 0.022 0.001 520 nm 0.075 0.073 0.0670.058 0.041 0.019 0.001 650 nm 0.116 0.114 0.107 0.097 0.079 0.055 0.036

[0037] After forming emission layer 106, the substrate temperature isagain increased to 1050° C. to successively grow p-typeAl_(0.2)Ga_(0.8)N blocking layer 107 of 20 nm thickness, p-type GaNlight guide layer 108 of 0.1 μm thickness, p-type Al_(0.1)Ga_(0.9)Ncladding layer 109 of 0.5 μm thickness and p-type GaN contact layer 110of 0.1 μm thickness. It is noted that Mg may be added at a concentrationof 5×10¹⁹-2×10²⁰/cm³ using EtCP2Mg (bisethyl cyclopentadienyl magnesium)as a p-type impurity.

[0038] The p-type impurity concentration in p-type GaN contact layer 110is preferably increased as it is closer to a junction surface withp-type electrode 112. Thus, the contact resistance with the p-typeelectrode may be reduced. Furthermore, in order to remove residualhydrogen which inhibits activation of Mg as a p-type impurity in thep-type layer, a small amount of oxygen may be introduced during growthof the p-type layer.

[0039] After growing p-type GaN contact layer 110, the entire gas in thereactor of the MOCVD apparatus is replaced with nitrogen carrier gas andNH₃ and then the substrate temperature is decreased at a rate of 60°C./min. At the time when the substrate temperature drops to 800° C., thesupply of NH₃ is stopped and the substrate temperature is maintained at800° C. for five minutes and is then decreased to the room temperature.It is noted that such a temperature temporarily held is preferablywithin 650° C. to 900° C., preferably for three to ten minutes. Thecooling rate from the held temperature to the room temperature ispreferably not less than 30° C./min.

[0040] Evaluation of the surface of the grown film by Raman measurementshowed p-type characteristics immediately after the growth even withoutsuch annealing for attaining p-type conductivity as used in aconventional nitride semiconductor film. Furthermore, the contactresistance was reduced when p-type electrode 112 was formed.

[0041] When the emission layer contains As, P or Sb as in the presentinvention, the emission layer is liable to be easily damaged by heat (areduced emission intensity, color unevenness and the like due to thephase separation) and it is undesirable to hold the substrate at atemperature higher than the growth temperature of the emission layer inan atmosphere other than NH₃ as it incurs a reduced emission intensity.Therefore, the technique of attaining p-type conductivity in the processof removing the substrate from the MOCVD apparatus as described aboveallows omission of annealing for p-type conductivity after removing thesubstrate and thus is very useful. It is noted that if the conventionalannealing for attaining p-type conductivity is not omitted, anactivation ratio of Mg is improved but the annealing time needs to beshortened (not more than ten minutes) at highest at the growthtemperature of the emission layer or lower (approximately not more than900° C.) in consideration of the damage of the emission layer. On theother hand, if annealing is performed after the side surface of emissionlayer 106 (904) (except a light radiation end surface in the case of alaser diode) is covered with dielectric film 113 (910) as shown in FIG.1 (or FIG. 7 or FIGS. 9(a) and 9(b)), escape of nitrogen or As (or P orSb) from the emission layer can be prevented and the phase separationand segregation in the emission layer can be suppressed.

[0042] It is then described how to process the epitaxial wafer taken outof the MOCVD apparatus into laser diode devices.

[0043] A part of n-type GaN contact layer 102 is first exposed using areactive ion etching apparatus, and n-type electrode 111 of stackedlayers of Hf/Au in this order is formed on this exposed part. For thisn-type electrode 111, stacked layers of Ti/Al, Ti/Mo, Hf/Al or the likecan also be used. Hf is effective in reducing the contact resistance ofthe n-type electrode. In the p-type electrode portion, etching isperformed in a striped manner along a <1-100> direction of sapphiresubstrate 100, SiO₂ dielectric film 113 is deposited, p-type GaN contactlayer 110 is exposed and stacked layers of Pd/Au in this order aredeposited, so as to form p-type electrode 112 having ridge stripe with awidth of 2 μm. For this p-type electrode, stacked layers of Ni/Au,Pd/Mo/Au or the like can also be used.

[0044] Finally, a Fabry-Perot resonator having a resonator length of 500μm is fabricated using cleavage or dry-etching. Preferably, theresonator length is generally within a range from 300 μm to 1000 μm.Mirror end surfaces of the resonator are formed to correspond to theM-plane of the sapphire substrate (see FIG. 2). Cleavage and laser chipdivision are carried out along dashed lines 2A and 2B in FIG. 2 from thesubstrate side using a scriber. In this manner, the flatness of thelaser end surfaces can be attained and at the same time shavings causedfrom scribing are not adhered on the surface of the epitaxial layer, andas a result it becomes possible to obtain a good yield of the lightemitting devices.

[0045] It is noted that the feed-back system of the laser resonator isnot limited to a Fabry-Perot type and may include a generally-known DFB(Distribution Feed Back) type, a DBR (Distribution Bragg Reflection)type and the like, as a matter of course.

[0046] After forming the mirror end surfaces of the Fabry-Perotresonator, dielectric films of SiO₂ and TiO₂ are alternately depositedon each of the mirror end surfaces to form a dielectric multilayerreflection film having a refractive index of 70%. A multilayer formed ofSiO₂/Al₂O₃ and the like can also be used as this dielectric multilayeredreflection film.

[0047] It is noted that a portion of n-type GaN contact layer 102 isexposed using reactive ion etching because an insulative sapphiresubstrate 100 is used. Therefore, when such a substrate havingconductivity as a GaN substrate or SiC substrate is used, a portion ofn-type GaN layer 102 needs not be exposed and an n-type electrode may beformed on the rear surface of that conductive substrate. Furthermore,although a plurality of n-type layers, an emission layer and a pluralityof p-type layers are successively crystal-grown from the substrate sidein the embodiment above, a plurality of p-type layers, an emission layerand a plurality of n-type layers may be successively crystal-grown inthe reversed order.

[0048] A method of packaging the laser diode chip as described abovewill now be described. First, if the laser diode including the emissionlayer as described above is used as a high power (50 mW) laser of violet(410 nm wavelength) suitable for a high density recording optical diskbecause of its characteristics, In solder material, for example, ispreferably used to connect the chip to a package body with asemiconductor junction downward. Alternatively, instead of attaching thechip directly to the package body or a heat sink portion, such asubmount having a good heat conductivity as of Si, AlN, diamond, MO,CuW, BN, SiC, Fe or Cu may be interposed for joint.

[0049] On the other hand, if the nitride semiconductor laser diodeincluding the emission layer as described above is formed on an SiCsubstrate, a nitride semiconductor substrate (for example, GaNsubstrate) or a GaN thick-film substrate (for example, the one formed bygrinding and removing a seed substrate 801 from a substrate 800 shown inFIG. 8) having a good heat conductivity, In solder material, forexample, can be used to connect the diode and the package body with thesemiconductor junction upward. Also in this case, instead of attachingthe substrate of the chip directly to the package body or the heat sinkportion, such a submount as of Si, AlN, diamond, Mo, CuW, BN, SiC, Fe,Cu may be interposed for connection.

[0050] In this way, it is possible to fabricate a laser diode usingnitride semiconductor containing As (or P or Sb) for the well layersincluded in the emission layer.

[0051] Further detailed description will now be made in conjunction withemission layer 106 included in the laser diode in the embodiment above.

[0052] Since the effective mass of electrons and holes is smaller in thewell layer of the nitride semiconductor containing As, P or Sb as in thepresent invention as compared with the InGaN well layer currently inpractical use, the laser threshold current density can be lowered ascompared with the conventional case using the InGaN layer. This leads torealization of a laser device with lower power consumption, high outputpower and a longer life thereof.

[0053] If a mixed crystal ratio of As, P or Sb is higher in the nitridesemiconductor crystal, however, phase separation of a hexagonal systemhaving a high N concentration and a cubic system (zinc blende structure)occurs in the crystal. In order to suppress this, the well layer and thebarrier layer included in the emission layer need to be preparedproperly.

[0054] First, the well layer has to be grown at a temperature of notless than 700° C. and not more than 900° C. This is because the welllayer containing As, P or Sb easily causes phase separation outside thisgrowth temperature range.

[0055] Furthermore, the thickness of the well layer is preferably withina range of 0.4-20 nm. This is because in the case of a low content ratioof As, P or Sb in the well layer for example for a wavelength band ofviolet (around 400 nm), a region of phase separation can be suppressedto 3% or less if the thickness of the well layer is not more than 20 nm.In the case of a high content ratio of As, P or Sb in the well layer forexample for a wavelength band of red (around 650 nm), a region havingphase separation can be suppressed to 3% or less if the thickness of thewell layer is not more than 5 nm. On the other hand, the thickness ofthe well layer needs to be not less than 0.4 nm, because the well layerhaving a thickness smaller than this value will not act as the emissionarea.

[0056] As to proper preparation of the barrier layer, first, in order tograsp the aforementioned phase separation phenomenon more precisely, anemission layer including GaNAs well layers/GaN barrier layers wasfabricated and the interface of the emission layer was observed by atransmission electron microscope (TEM). As a result, phase separationwas observed more significantly at an interface of the GaN layer on theGaNAs layer than at an interface of the GaNAs layer on the GaN layer.Furthermore, as the number of stacked well layers and barrier layers wasincreased, this tendency became more conspicuous, and the emission layermostly suffered the phase separation in an area close to a epitaxialgrowth outermost surface.

[0057] Based on such a fact, it can be considered that the phaseseparation in the GaNAs well layer propagates one after another throughthe barrier layer on the well layer in contact therewith and the entirearea of the emission layer suffers phase separation in the vicinity ofthe outermost surface. This suggests that in such an emission layer, itis difficult to form a multiple quantum well structure with a pluralityof well layers and barrier layers alternately stacked. Furthermore,considering that phase separation is significant at the interface of theGaN layer on the GaNAs layer, an absorption ratio of Ga to As ispresumably higher than that of Ga to N when the GaN layer is grown onthe GaNAs layer. In addition, although it is basically preferable thatthe GaN crystal is grown at 1000° C. or higher, the GaN layer has to begrown at the same temperature as the well layer in order to suppress thephase separation in the well layer, and then it is considered thatlowered crystallinity of GaN presumably increases the absorption ratioof Ga to As. In further addition, although the temperature needs to beincreased to 1000° C. or higher in growing the p-type layer on theemission layer, such a high temperature is more likely to cause Assegregation on the surface of the GaNAs well layer, and phase separationis presumably induced by internal diffusion of As at the interfacebetween the well layer and the barrier layer.

[0058] The phase separation as described above may similarly occur alsoin another nitride semiconductor well layer containing As, P or Sb as ina well layer of GaNP, GaNSb, GaNAsPSb, InGaNAs, InGaNP, InGaNSbInGaNAsPSb or the like. It is noted that a well layer containing atleast As, P or Sb and also containing Al may attain an effect similar tothat in a barrier layer according to the present invention describedlater and may preferably be used. For example, AlInGaNAs, AlInGaNP,AlInGaNSb, AlInGaNAsPSb or the like may be used for a well layer.

[0059] In order to suppress phase separation in the well layer, abarrier layer of nitride semiconductor containing Al is desired. Al hashigh vapor-phase reactivity. Therefore, before Al atoms reach thesurface of the epitaxial wafer, they mostly forms nitride semiconductorcrystalline nuclei containing Al to deposit on the wafer surface. SinceAl has strong chemical binding force, when the stable barrier layer isstacked on the well layer containing As, P or Sb, recrystallization dueto binding with As (or P or Sb) does not take place. Furthermore, thestable barrier layer may also act to prevent the escape of As (or P orSb) or N from the well layer.

[0060] In addition, even if the substrate temperature is increased to atemperature higher than the growth temperature of the well layer (1000°C. or higher) in order to stack a p-type layer, diffusion into thebarrier layer due to segregation of As (or P or Sb) hardly occurs sincethe nitride semiconductor crystal containing Al can be stably present.Therefore, the barrier layer containing Al may act to preventpropagation of influence of the phase separation from one well layer toanother well layer. In other words, the use of the barrier layercontaining Al makes it possible to fabricate a multiple quantum wellstructure.

[0061] Generally, the nitride semiconductor crystal containing Al,however, has poor crystallinity unless the growth temperature isincreased (to 900° C. or higher). However, the growth temperature of thebarrier layer can be decreased to the same level as the growthtemperature of the well layer by adding any of elements As, P, Sb and Ininto the barrier layer. Thus, phase separation in the emission layerresulting from a high growth temperature of the barrier layer can beprevented and at the same time the crystallinity of the barrier layercan be improved. The respective contents of As, P and Sb in the group Velements in such a barrier layer are preferably not more than 7.5% forAs, not more than 10% for P and not more than 2.5% for Sb. This isbecause if the content of As, P or Sb is too high, phase separationstarts to occur even with the barrier layer containing Al. Furthermore,In content in the group III elements in the barrier layer may be notmore than 15%. This is because concentration separation, which wasobserved in the conventional InGaN, is hardly seen with not more than15% of In, as the barrier layer contains Al.

[0062] Furthermore, In may be further added into the barrier layercontaining element Al and As, P or Sb. This is because inclusion of Inreduces the energy band gap in the barrier layer so that the content ofAs, P or Sb can be reduced.

[0063] The thickness of the barrier layer is preferably not less than 1nm and not more than 20 nm. Although the thickness of the barrier layeris preferably equal to or smaller than that of the well layer in orderto form a subband by a multiple quantum well structure, it is preferablyequal to or slightly larger than that of the well layer in order toblock the influence of the phase separation in the well layer.

[0064] As to addition of the impurity in the emission layer, althoughSiH₄ (Si) is added to both the well layer and the barrier layer as animpurity in this embodiment, laser excitation is possible even when theimpurityi is added only to either of the layers or to neither of thelayers. According to a photoluminescence (PL) measurement, however, thePL emission intensity is about 1.2 to 1.4 times stronger in the casewhere SiH₄ is added to both the well layer and the barrier layer ascompared with the case where it is not added. Thus, an impurity such asSiH₄ (Si) is preferably added in the emission layer in the lightemitting diode. Since the well layer according to the present inventionis formed as a mixed crystal system including As, P or Sb, localizedenergy levels due to In are less likely to be formed as compared withInGaN crystal including no As, P and Sb even though In is contained inthe well layer. Therefore, it can be considerd that the emissionintensity has strong dependency on the crystallinity of the well layer.Thus, it is necessary to improve the crystallinity of the emission layerby adding an impurity such as Si. More specifically, the crystallinityis improved by producing nuclei for crystal growth by such an impurityand crystal-growing the well layer based on the nuclei. Although Si(SiH₄) is added at a concentration of 1×10¹⁸/cm³ in this embodiment, O,S, C, Ge, Zn, Cd, Mg or the like other than Si may also be added toattain a similar effect. The concentration of these added atoms ispreferably about 1×10¹⁶-1×10²⁰/cm³.

[0065] Generally, in the case of a laser diode, modulation dope in whichan impurity is added only to the barrier layer can reduce laserexcitation threshold current density because of no carrier absorption inthe well layer. On the contrary, the threshold value was lower when animpurity is added in the well layer according to the present invention.Presumably, this is because the crystal growth in the present embodimentstarting from a sapphire substrate different from a nitridesemiconductor substrate causes many crystal defects (threadingdislocation density is about 1×10¹⁰/cm²), and thus improving thecrystallinity by addition of the impurity is more effective in reducingthe laser threshold current density rather than considering carrierabsorption by the impurity in the well layer.

[0066] In FIG. 3, relation between the number of well layers included inthe emission layer (multiple quantum well structure) and the laserthreshold current density are relatively compared between use of asapphire substrate and use of a GaN substrate. In this graph, theabscissa represents the number of well layers and the ordinaterepresents the threshold current density. Furthermore, ◯ mark representsthe laser threshold current density in the case of using a sapphiresubstrate and  mark represents that in the case of using a GaNsubstrate. When the number of well layers is not more than ten, thethreshold current density is low and a continuous emittion at the roomtemperature is possible. In order to further reduce the laser excitationthreshold current density, the number of well layers is preferably notless than two and not more than five. Furthermore, the threshold currentdensity is found to be lower when a GaN substrate is used rather than asapphire substrate.

[0067] On emission layer 106, p-type AlGaN blocking layer 107 and p-typelayer 108 are provided to stack in this order. This p-type layer 108corresponds to a p-type light guide layer in a laser diode andcorresponds to a p-type cladding layer or a p-type contact layer in alight emitting diode.

[0068] According to the PL measurement, in comparison between thepresence and the absence of blocking layer 107, a shift amount from adesigned emission wavelength was smaller and the PL emission intensitywas stronger in the presence of the blocking layer. As described inconnection with the emission layer of the laser diode above, as comparedwith emission layer 106, p-type layer 108 thereover is formed at ahigher growth temperature and thus acts to assist phase separation inthe emission layer. It is considered, however, that the phase separationin the emission layer may be suppressed and the influence from emissionlayer 106 containing As, P or Sb (for example, phase separation) can beprevented from propagating to p-type layer 108 by providing blockinglayer 107 containing Al at the interface between the emission layer andthe p-type layer. Particularly, when emission layer 106 has a structureshown in FIG. 4(b) in which emission layer having a multiple quantumwell structure starts at the well layer and ends at the well layer, theeffect of blocking layer 107 is noticeable.

[0069] In light of the foregoing, it is important that blocking layer107 contains at least Al. Furthermore, the polarity of the blockinglayer is preferably p-type. This is because unless the blocking layer isp-type, the position of a pn junction in the vicinity of the emissionlayer changes, causing reduced luminous efficiency.

[0070] Similarly to the case described above, an n-type AlGaN blockinglayer may be provided between emission layer 106 and n-type layer 105 tobe into contact therewith. This n-type layer 105 corresponds to ann-type light guide layer in the case of a laser diode and corresponds toan n-type cladding layer or an n-type contact layer in the case of alight emitting diode. The effect of such an n-type AlGaN blocking layeris approximately similar to that of p-type AlGaN blocking layer 107.

[0071] As to the band gap structure of the emission layer, FIG. 6 showsa conventional band gap structure (Japanese Patent Laying-Open No.10-270804) and FIG. 4(a) shows the band gap structure of thisembodiment. In the conventional band gap structure shown in FIG. 6, acladding layer is formed of AlGaN, a light guide layer is formed of GaN,a barrier layer is formed of GaN and a well layer is formed of GaNAs. Asthe light guide layer and the barrier layer are made of the same nitridesemiconductor material, they have the same energy band gap andrefractive index. Therefore, this case hardly causes the subband due tothe multiple quantum well effect but causes a decrease in a gain (anincrease in the threshold current density) in the case of a laser diodeand an increase in a half-width of wavelength (which causes colorirregularity) in the case of a light emitting diode. Furthermore, in thecase of the AlGaN cladding layer/GaN light guide layer, a refractiveindex difference therebetween is originally small and the barrier layeris also formed of GaN, so that the light confinement efficiency is smalland the vertical lateral mode characteristics are not good.

[0072] Therefore, in this embodiment as shown in FIG. 4(a), the energyband gap of the barrier layer is made small as compared with the lightguide layer. Accordingly, the multiple quantum well effect causing thesubband is more easily obtained as compared with the conventionalembodiment shown in FIG. 6, and the light confinement efficiency isimproved with the refractive index of the barrier layer larger than thatof the light guide layer, causing improvement of the vertical lateralmode characteristics (attaining a unimodal mode). Particularly, when thebarrier layer contains As, P or Sb, the increasing tendency of therefractive index is preferably noticeable.

[0073] The emission layer where the energy band gap of the barrier layeris smaller than that of the light guide layer as describe above canemploy two kinds of configurations as shown in FIGS. 4(a) and 4(b).Specifically, the emission layer having a multiple quantum wellstructure may employ either of a configuration starting from a barrierlayer and ending at a barrier layer or a configuration starting from awell layer and ending at a well layer. The possible band gap structuresof the emission layer without a blocking layer are as shown in FIGS.5(a) and 5(b).

[0074] (Embodiment 2)

[0075] Embodiment 2 uses a variety of nitride semiconductor materialsfor the well layer and the barrier layer in the emission layer having amultiple quantum well structure as described in embodiment 1. Thecombinations of nitride semiconductor materials for the well layer andthe barrier layer are shown in Table 3. TABLE 3 barrier layer AlGaNAlGaNAs AlGaNP AlGaNSb InAlGaN InAlGaNAs InAlGaNP InAlGaNSb well layerGaNAs Δ ⊚ ⊚ ⊚ ⊚ ⊚ ⊚ ⊚ GaNP Δ ⊚ ⊚ ⊚ ⊚ ⊚ ⊚ ⊚ GaNSb Δ ⊚ ⊚ ⊚ ⊚ ⊚ ⊚ ⊚ InGaNAsΔ ⊚ ⊚ ⊚ ⊚ ⊚ ⊚ ⊚ InGaNP Δ ⊚ ⊚ ⊚ ⊚ ⊚ ⊚ ⊚ InGaNSb Δ ⊚ ⊚ ⊚ ⊚ ⊚ ⊚ ⊚ AlGaNAs Δ⊚ ⊚ ⊚ ⊚ ⊚ ⊚ ⊚ AlGaNP Δ ⊚ ⊚ ⊚ ⊚ ⊚ ⊚ ⊚ AlGaNSb Δ ⊚ ⊚ ⊚ ⊚ ⊚ ⊚ ⊚ InAlGaNAs Δ⊚ ⊚ ⊚ ⊚ ⊚ ⊚ ⊚ InAlGaNP Δ ⊚ ⊚ ⊚ ⊚ ⊚ ⊚ ⊚ InAlGaNSb Δ ⊚ ⊚ ⊚ ⊚ ⊚ ⊚ ⊚

[0076] In Table 3, Δ mark shows a less preferable combination of nitridesemiconductor materials for the well layer and the barrier layer and ⊚mark shows a preferable combination. It is noted that only an AlGaNbarrier layer is less preferable in Table 3 for the reason as follows.Specifically, as described above, unless the growth temperature of thewell layer of the present invention is not less than 700° C. and notmore than 900° C., phase separation of different crystal systems occursin the well layer. However, since a proper growth temperature of theAlGaN barrier layer is not less than 900° C., phase separation occurs inthe well layer if the barrier layer is grown at that proper growthtemperature. On the other hand, if the barrier layer is grown at agrowth temperature proper for the well layer (not less than 700° C. andnot more than 900° C.), crystallinity of the AlGaN barrier layer isundesirably deteriorated. The only temperature suitable for both theAlGaN barrier layer and the well layer is 900° C., so that the crystalgrowth range is narrow and controllability is poor.

[0077] Though the well layer contains any of elements As, P and Sb inTable 3, it may contain these plural kinds of elements. Specifically itmay be a mixed crystal of XGaN_(1-x-y)As_(x)P_(y)Sb_(z) (0≦x≦0.15,0≦y≦0.2, 0≦z≦0.05, x+y+z>0), where X represents one or more kinds ofgroup III elements. It is noted that the other conditions for theemission layer using these nitride semiconductor materials are similarto those in embodiment 1.

[0078] (Embodiment 3)

[0079] In embodiment 3 shown in FIG. 7, n-type GaN substrate 700 havinga C-plane ({0001} plane) as a main surface was used in place of sapphiresubstrate 100 used in embodiment 1. When the GaN substrate is used, GaNbuffer layer 101 may be omitted and n-type GaN layer 102 may be growndirectly on the GaN substrate. Since the GaN substrate commerciallyavailable at present does not have good enough crystallinity and surfacemorphology, however, GaN buffer layer 101 is preferably inserted inorder to improve these.

[0080] Since n-type GaN substrate 700 is used in embodiment 3, n-typeelectrode 111 can be formed on the rear surface of GaN substrate 700.Furthermore, since the GaN substrate can have a very flat. cleavage endsurface, a Fabry-Perot resonator having a resonator length of 300 μm canbe fabricated with a small mirror loss. It is noted that similarly toembodiment 1 the resonator length is preferably within a range from 300μm to 1000 μm in general. The mirror end surfaces of the resonator areformed to correspond to {1-100} plane of GaN substrate 700. The cleavageand division for laser chips are performed from the substrate side usinga scriber as in FIG. 2 explained above. Furthermore, the aforementionedDFB or DBR may be used as a feed-back system of the laser resonator, asa matter of course. It is needless to say that a dielectric multilayerreflection film similar to that of the first embodiment may be formed onthe mirror end surface.

[0081] By using a GaN substrate in place of a sapphire substrate, thethicknesses of n-type AlGaN cladding layer 104 and p-type AlGaN claddinglayer 109 can be increased without causing cracks in the epitaxialwafer. Preferably, the thickness of the AlGaN cladding layer is setwithin 0.8 μm to 1.8 μm. As a result, the umimodal vertical lateral modeand the light confinement efficiency are improved, the optical propertyof the laser device can be improved, and the laser threshold currentdensity can be reduced.

[0082] The characteristics of the well layer included in the emissionlayer according to the present invention have strong dependency on thecrystallinity (crystal defects) of the well layer as described above.Therefore, if a nitride semiconductor laser diode device including thewell layer is fabricated using a GaN substrate as in the presentembodiment, the crystal defect density in the emission layer (forexample, threading dislocation density) is reduced and thus the laserexcitation threshold current density is reduced by 10% to 20% ascompared with embodiment 1 using a sapphire substrate (see FIG. 3).

[0083] It is noted that the other conditions for the emission layer inthe present embodiment are similar to those in embodiment 1. As toimpurity concentration in the emission layer, however, the laserthreshold current density was reduced as compared with embodiment 1 bymodulation dope in which an impurity is added only in the barrier layeror by adding an impurity at a concentration of not more than 3×10¹⁸/cm³to the well layer. This is because the crystallinity of the emissionlayer was presumably improved as compared with the case using thesapphire substrate.

[0084] (Embodiment 4)

[0085] Embodiment 4 is similar to embodiment 1 or 3 except that sapphiresubstrate 100 in embodiment 1 was replaced by a substrate 800 shown inFIG. 8. Substrate 800 in FIG. 8 includes a seed substrate 801, a bufferlayer 802, an n-type GaN film 803, a dielectric film 804 and an n-typeGaN thick film 805, which are successively stacked.

[0086] In fabricating substrate 800, buffer layer 802 is first stackedon seed substrate 801 by MOCVD at a relatively low temperature of 550°C. Then n-type GaN film 803 of 1 μm thickness is formed thereon withdoping of Si at a temperature of 1050° C.

[0087] The wafer including seed substrate to n-type GaN film 803 istaken out of the MOCVD apparatus and then dielectric film 804 is formedin a thickness of 100 nm using sputtering, CVD or EB deposition.Dielectric film 804 is processed into a periodic striped pattern by alithography technique. These stripes are along <1-100> direction ofn-type GaN film 803 and has a periodic pitch of 10 μm and a width of 5μm in a <11-20> direction perpendicular to <1-100>.

[0088] The wafer including dielectric film 804 processed in a stripedpattern is placed in an HVPE apparatus, and n-type GaN thick film 805having an Si concentration of 1×10¹¹/cm³ and a thickness of 350 μm isdeposited at a growth temperature of 1100° C.

[0089] The wafer including n-type GaN thick film 805 was taken out ofthe HVPE apparatus, and a laser diode similar to embodiment 1 (seeFIG. 1) was fabricated thereon. In embodiment 4, however, a ridge stripeportion 1A of the laser diode was formed so as not to be positioned juston lines 810 and 811 in FIG. 8. This is because it is preferable thatthe laser device is fabricated selectively on a portion having a lowthreading dislocation density (crystal defect density). Thecharacteristics of the laser diode thus fabricated in embodiment 4 werebasically similar to those of embodiment 3.

[0090] It is noted that substrate 800 from which seed substrate 801 hasbeen removed by a grinder may also be used as a substrate for a laserdiode. Furthermore, substrate 800 from which buffer layer 802 and allthe lower layers have been removed by a grinder may also be used as alaser diode substrate. Still further, substrate 800 from whichdielectric film 804 and all the lower layers have been removed by agrinder may also be used as a substrate for a laser diode. If seedsubstrate 801 is removed, n-type electrode 111 can be formed on the rearsurface of the substrate similarly as in embodiment 3. It is noted thatseed substrate 801 may be removed after fabricating the laser diodestructure over substrate 800.

[0091] In fabricating the aforementioned substrate 800, any of C-planesapphire, M-plane sapphire, A-pane sapphire, R-plane sapphire, GaAs,ZnO, MgO, Spinel, Ge, Si, 6H-SiC, 4H-SiC, 3C-SiC and the like may beused as seed substrate 801. Any of a GaN layer, an AN layer, anAl_(x)Ga_(1-x)N (0≦x≦1) layer or an In_(y)Ga_(1-y)N (0≦y≦1) layer grownat a relatively low temperature of 450° C. to 600° C. may be used asbuffer layer 802. Any of an SiO₂ film, an SiN_(x) film, a TiO₂ film oran Al₂O₃ film may be used as dielectric film 804. N-type GaN thick film805 may be replaced with n-type type Al_(w)Ga_(1-w)N (0<w≦1) thick filmof which thickness may be not less than 20 μm.

[0092] (Embodiment 5)

[0093] In embodiment 5, the materials of the light guide layer ofembodiment 1 were varied. Although both of n-type light guide layer 105and p-type light guide layer 108 were formed of GaN in embodiment 1, apart of nitrogen atoms of these GaN layers may be replaced with any ofelement As, P or Sb. Specifically, a light guide layer ofGaN_(1-x-y)As_(x)P_(y)Sb_(z) (0≦x≦0.075, 0≦y≦0.1, 0≦z≦0.025, x+y+z>0)can be used.

[0094] In the conventional AlGaN cladding layer/GaN light guide layer,even if the Al content in the cladding layer is increased, therefractive index difference between these layers is small and latticemismatch is increased, leading to cracking or deterioratedcrystallinity. On the other hand, in the case of a combination of anAlGaN cladding layer and a GaNAsPSb light guide layer, an energy gapdifference as well as a refractive index difference are increased with asmall amount of lattice mismatch as compared with the conventionalembodiment, due to significant bowing effect in the band gap caused byAs, P or Sb. Thus, laser light can efficiently be confined and verticallateral mode characteristics (a unimodal mode) are improved in thenitride semiconductor laser diode device.

[0095] As to a composition ratio in the GaN_(1-x-y-z)As_(x)P_(y)Sb_(z)(0≦x≦0.075, 0≦y≦0.1, 0≦z≦0.025, x+y+z>0) light guide layer, acomposition ratio of x, y and z may be adjusted such that the lightguide layer has a larger energy band gap as compared with the barrierlayer in the emission layer. For example, in the GaN_(1-x)As_(x) lightguide layer in a violet laser (410 nm wavelength) device, a compositionratio x of As is adjusted to not more than 0.02, in the case of theGaN_(1-y)P_(y) light guide layer, a composition ratio y of P is adjustedto not more than 0.03, and in the case of the GaN_(1-z)Sb_(z) lightguide layer, a composition ratio z of Sb is adjusted to not more than0.01. It is noted that the other conditions for the emission layer inembodiment 5 are similar to those in embodiment 1.

[0096] (Embodiment 6)

[0097] Embodiment 6 is for a nitride semiconductor light emitting diodedevice. FIG. 9(a) is a longitudinal sectional view and FIG. 9(b) is atop view schematically showing the nitride semiconductor light emittingdiode device in embodiment 6.

[0098] The light emitting diode device in FIG. 9(a) includes a C-plane(0001) sapphire substrate 900, a GaN buffer layer 901 (30 nm thick), ann-type GaN layer contact 902 (3 μm thick, Si impurity concentration1×10¹⁸/cm³), an n-type Al_(0.1)Ga_(0.9)N blocking and cladding layer 903(20 nm thick, Si impurity concentration 1×10¹⁸/cm³), an emission layer904, a p-type Al_(0.1)Ga_(0.9)N blocking and cladding layer 905 (20 nmthick, Mg impurity concentration 6×10¹⁹/cm³), a p-type GaN contact layer906 (200 nm thick, Mg impurity concentration 1×10²⁰/cm³), a transparentp-type electrode 907, a pad electrode 908, an n-type electrode 909 and adielectric film 910.

[0099] It is noted that n-type Al_(0.1)Ga_(0.9)N blocking and claddinglayer 903 may be omitted in such a light emitting diode device.Furthermore, p-type electrode 907 may be formed of Ni or Pd, padelectrode 908 may be formed of Au, and n-type electrode 909 may beformed of a stack of Hf/Au, Ti/Al, Ti/Mo or Hf/Al.

[0100] In the emission layer of the present embodiment, SiH₄ (Siimpurity concentration of 5×10¹⁷/cm³) is added respectively to the welllayer and the barrier layer. It is noted that the nitride semiconductormaterials for these well and barrier layers are similar to those inembodiment 1. Furthermore, if a GaN substrate is used in place of thesapphire substrate, an effect similar to that in embodiment 3 isacheived, and if a substrate shown in FIG. 8 is used, an effect similarto that in embodiment 4 is achieved. Furthermore, as the GaN substrateis conductive, both of p-type electrode 907 and n-type electrode 909 maybe formed on one side of the light emitting device as shown in FIG.9(b), while an n-type electrode may be formed on the rear surface of theGaN substrate and a transparent p-type electrode may be formed on theepitaxial outermost surface.

[0101] It is noted that the conditions for the well layer and thebarrier layer included in emission layer 904 in this embodiment 6 aresimilar to those in embodiment 1.

[0102]FIG. 10 shows relation between the emission intensity and thenumber of the well layers included in the emission layer of the lightemitting diode device. More specifically, in this graph, the abscissarepresents the number of well layers and the ordinate shows theemission-intensity (arb.units: normalized arbitrary unit). In otherwords, in FIG. 10, the emission intensity of the light emitting diode isnormalized on the basis of the conventional InGaN well layer (brokenline) used in place of the GaNP well layer (alternatively a GaNAs welllayer or a GaNSb well layer). In the graph, ◯ mark shows the emissionintensity in the case of a sapphire substrate and  mark shows theemission intensity in the case of the GaN substrate. It can beunderstood from this graph that the preferable number of well layersincluded in the light emitting diode is not less than two and not morethan ten. It can also be understood that the emission intensity isimproved using the GaN substrate rather than using the sapphiresubstrate.

[0103] (Embodiment 7)

[0104] Embodiment 7 is for a nitride semiconductor super-luminescentdiode device. The configuration and crystal growth method is similar tothat described in embodiment 1. It is noted that if a GaN substrate isused in place of the sapphire substrate, an effect similar to embodiment3 is achieved and if the substrate shown in FIG. 8 is used, an effectsimilar to embodiment 4 is achieved. Furthermore, relation between theemission intensity and the number of well layers included in theemission layer is similar to that shown in embodiment 6.

[0105] (Embodiment 8)

[0106] In embodiment 8, C of 1×10²⁰/cm³ was added in place of theimpurity Si to the well layer and the barrier layer in the emissionlayer shown in embodiments 1, 3, 4, 6 and 7. In this manner, C used inplace of the impurity Si in the well layer and the barrier layer causedthe similar effect.

[0107] (Embodiment 9)

[0108] In embodiment 9, Mg of 1×10¹⁶/cm³ was added in place of Si as animpurity to the well layer and the barrier layer in the emission layershown in embodiments 1, 3, 4, 6 and 7. In this manner, Mg used in placeof Si as an impurity in the well layer and the barrier layer caused thesimilar effect.

[0109] (Embodiment 10)

[0110] In embodiment 10, although the well layers and the barrier layersincluded in the emission layer shown in embodiments 1, 3, 4, 6 and 7were changed to five-cycle GaN_(0.98)P_(0.02) well layers (2 nmthick)/Al_(0.01)In_(0.06)Ga_(0.93)N barrier layers (4 nm thick), aneffect similar to that in each embodiment was achieved.

[0111] (Embodiment 11)

[0112] Embodiment 11 differs from each of embodiments 1, 3, 4, 6 and 7only in that the well layers and the barrier layers included in theemission layer were changed to ten-cycle GaN_(0.95)Sb_(0.05) well layers(0.4 nm thick)/GaN barrier layers (1 nm thick, Al impurity concentrationof 5×10¹⁸/cm³). The PL measurement was performed on the light emittingdevice of embodiment 11 and the conventional light emitting device.While a plurality of emission wavelength peaks resulting from phaseseparation in the emission layer was observed in the conventional deviceincluding GaN barrier layers free of Al, only one emission wavelengthpeak was observed in the device of embodiment 11. This suggests thatphase separation in the emission layer was suppressed in the lightemitting device according to embodiment 11.

[0113] (Embodiment 12)

[0114] In embodiment 12, although the well layers and the barrier layersincluded in the emission layer shown in embodiments 1, 3, 4, 6 and 7were changed to two-cycle GaN_(0.97)As_(0.03) well layers (6 nmthick)/In_(0.04)Al_(0.02)Ga_(0.94)N_(0.9)P_(0.01) barrier layers (6 nmthick), an effect similar to that of each of embodiments 1, 3, 4, 6 and7 was achieved.

[0115] (Embodiment 13)

[0116] In embodiment 13, although the well layers and the barrier layersincluded in the emission layer shown in embodiments 1, 3, 4, 6 and 7were changed to four-cycle GaN_(0.98)As_(0.02) well layers (4 nmthick)/Al_(0.01)Ga_(0.9)No_(0.99)As_(0.01) barrier layers (10 nm thick),an effect similar to that of each of embodiments 1, 3, 4, 6 and 7 wasachieved.

[0117] (Embodiment 14)

[0118] In embodiment 14, although the well layers and the barrier layersincluded in the emission layer shown in embodiments 1, 3, 4, 6 and 7were changed to three-cycle GaN_(0.97)P_(0.03) well layers (18 nmthick)/Al_(0.01)Ga_(0.99)N_(0.98)P_(0.02) barrier layers (20 nm thick),an effect similar to that of each of embodiments 1, 3, 4, 6 and 7 wasachieved.

[0119] (Embodiment 15)

[0120] In embodiment 15, although the well layers and the barrier layersincluded in the emission layer shown in embodiments 1, 3, 4, 6 and 7were changed to three-cycle GaN_(0.97)P_(0.03) well layers (5 nmthick)/Al_(0.1)Ga_(0.9)N_(0.94)P_(0.06) barrier layers (5 nm thick), aneffect similar to that of each of embodiments 1, 3, 4, 6 and 7 wasachieved.

[0121] (Embodiment 16)

[0122] In embodiment 16, although the well layers and the barrier layersincluded in the emission layer shown in embodiments 1, 3, 4, 6 and 7were replaced by three-cycle In_(0.05)Ga_(0.95)N_(0.98)P_(0.02) welllayers (4 nm thick)/Al_(0.01)In_(0.06)Ga_(0.93)N barrier layers (8 nm),an effect similar to that of each of embodiments 1, 3, 4, 6 and 7 wasachieved.

[0123] (Embodiment 17)

[0124] In embodiment 17, although the well layers and the barrier layersincluded in the emission layer shown in embodiments 1, 3, 4, 6 and 7were replaced by five-cycle In_(0.1)Ga_(0.9)N_(0.94)As_(0.6) well layers(2 nm)/Al_(0.01)In_(0.06)Ga_(0.93)N barrier layers (4 nm), an effectsimilar to that of each of embodiments 1, 3, 4, 6 and 7 was achieved.

[0125] (Embodiment 18)

[0126] In embodiment 18, although the well layer and the barrier layerincluded in the emission layer in embodiments 1, 3, 4, 6 and 7 werereplaced by five-cycle Al_(0.01)In_(0.1)Ga_(0.89)N_(0.94)As_(0.06)well-layers (2 nm)/Al_(0.01)In_(0.06)Ga_(0.93)N barrier layers (4 nm),an effect similar to that of each of embodiments 1, 3, 4, 6 and 7 wasachieved.

[0127] (Embodiment 19)

[0128] In embodiment 19, although the well layers and the barrier layersincluded in the emission layer shown in embodiments 1, 3, 4, 6 and 7were replaced by three-cycle Al_(0.01)In_(0.05)Ga_(0.94)N_(0.96)P_(0.04)well layers (4 nm)/Al_(0.01)In_(0.06)Ga_(0.93)N barrier layers (8 nm),an effect similar to that of each of embodiments 1, 3, 4, 6 and 7 wasachieved.

[0129] (Embodiment 20)

[0130] In embodiment 20, an optical apparatus using the nitridesemiconductor laser shown in embodiments 1 to 5 was fabricated. In anoptical apparatus, for example, using a violet (an emission wavelengthof 400-410 nm) nitride semiconductor laser according to the presentinvention, the laser excitation threshold current density is low ascompared with the conventional nitride semiconductor laser, spontaneousemission light in the laser light is reduced, and noise light is alsoreduced. Furthermore, such a laser device can stably operate at a highpower (50 mW) in a high temperature atmosphere, so that it is suitablefor a recording and reproducing optical apparatus used for a highdensity recording and reproducing optical disk.

[0131] In FIG. 11, an optical disk information recording and reproducingapparatus including an optical pickup device 2 is shown in a schematicblock diagram as an exemplary optical apparatus including a laser device1 according to the present invention. In this optical informationrecording and reproducing apparatus, laser light 3 is modulated inresponse to input information by an optical modulator 4 and theinformation is recorded on a disk 7 through a scan mirror 5 and a lens6. Disk 7 is rotated by a motor 8. In reproduction, reflected laserlight optically modulated by pit arrangement on disk 7 is detected by adetector 10 through a beam splitter 9, thereby producing a reproductionsignal. Operation in each of these components is controlled by a controlcircuit 11. The output of laser device 1 is normally 30 mW in recordingand about 5 mW in reproduction.

[0132] The laser device according to the present invention may be usednot only in the optical disk recording and reproducing apparatus asdescribed above but also in a laser printer, a projector using a laserdiode of three primary colors (blue, green and red), and the like.

[0133] (Embodiment 21)

[0134] In embodiment 21, the nitride semiconductor light emitting diodeaccording to embodiments 6 and 7 were used in an optical apparatus. Awhite light source including light emitting diodes or super-luminescentdiodes of three primary colors (red, green and blue) using the emissionlayer according to the present invention could be fabricated and adisplay using these three primary colors could also be fabricated.

[0135] Backlight with lower power consumption and high luminousintensity for use in a crystal liquid display can be obtained by usingsuch a white light source including the light emitting devices accordingto the present invention in place of a conventional halogen lightsource. More specifically, the white light source including the lightemitting devices according to the present invention can be used asbacklight for a liquid crystal display of man-machine interface for amobile notebook-sized personal computer, a mobile phone and the like andcan provide a liquid crystal display having a reduced size and sharppicture quality.

[0136] Incidentally, it is noted that the XN_(1-x-y-z)As_(x)P_(y)Sb_(z)well layer in the present invention has to satisfy the conditions ofx≦0.15, y≦0.2 and z≦0.05. This is because crystallinity of the welllayer is deteriorated unless these conditions are not satisfied.

INDUSTRIAL APPLICABILITY

[0137] As described above, according to the present invention, in anitride semiconductor light emitting device including an emission layerhaving a multiple quantum well structure where a plurality of quantumwell layers and a plurality of barrier layers are alternately stacked,the quantum well layer is formed of XN_(1-x-y-z)As_(x)P_(y)Sb_(z)(0≦x≦0.15, 0≦y≦0.2, 0≦z≦0.05, x+y+z>0) and Al is contained in thebarrier layer, whereby crystal phase separation in the well layer can besuppressed and a luminous efficiency of the light emitting device can beimproved. It is noted that X represents one or more kinds of group IIIelements.

1. A nitride semiconductor light emitting device comprising an emissionlayer having a multiple quantum well structure where a plurality ofquantum well layers and a plurality of barrier layers are alternatelystacked, wherein said quantum well layer is formed ofXN_(1-x-y-z)As_(x)P_(y)Sb_(z) (0≦x≦0.15, 0≦y≦0.2, 0≦z≦0.05, x+y+z>0),where X represents at least one kind of group III elements, and saidbarrier layer is formed of a nitride semiconductor layer containing atleast Al at a concentration of at least 5×10¹⁸/cm³ and having athickness of at least 1 nm and at most 20 nm.
 2. The nitridesemiconductor light emitting device according to claim 1, wherein saidbarrier layer further contains In.
 3. The nitride semiconductor lightemitting device according to claim 1, wherein said barrier layer furthercontains any element selected from the group consisting of As, P and Sb.4. The nitride semiconductor light emitting device according to claim 1,further comprising a substrate for growing a plurality of semiconductorlayers included in said nitride semiconductor light emitting device,wherein in a first adjacent semiconductor layer in contact with a firstmain surface of said emission layer closer to said substrate and asecond adjacent semiconductor layer in contact with a second mainsurface of said emission layer farther from said substrate, at leastsaid second adjacent semiconductor layer is formed of nitridesemiconductor containing at least Al.
 5. The nitride semiconductor lightemitting device according to claim 4, wherein one of said well layers isdirectly in contact with said first adjacent semiconductor layer or saidsecond adjacent semiconductor layer.
 6. (Cancelled)
 7. The nitridesemiconductor light emitting device according to claim 3, wherein ingroup V elements in said barrier layer, As content is at most 7.5%, Pcontent is at most 10% and Sb content is at most 2.5%.
 8. The nitridesemiconductor light emitting device according to claim 1, wherein saidemission layer includes at least two and at most ten of said welllayers.
 9. The nitride semiconductor light emitting device according toclaim 1, wherein said well layer has a thickness of at least 0.4 nm andat most 20 nm.
 10. (Cancelled)
 11. The nitride semiconductor lightemitting device according to claim 1, wherein at least one kind ofdopant selected from the group consisting of Si, O, S, C, Ge, Zn, Cd andMg is added to at least either said well layers or said barrier layers.12. The nitride semiconductor light emitting device according to claim11, wherein an added amount of said dopant is within a range of 1×10¹⁶to 1×10²⁰/cm³.
 13. The nitride semiconductor light emitting deviceaccording to claim 1, wherein said light emitting device is formed usinga GaN substrate.
 14. An optical apparatus comprising said nitridesemiconductor light emitting device according to claim
 1. 15. Thenitride semiconductor light emitting device according to claim 1,further comprising a striped mask formed on a substrate, said masksuppressing growth of nitride semiconductor crystal thereon, and anitride semiconductor stacked layer structure crystal-grown to coversaid substrate and said mask, wherein said nitride semiconductor stackedlayer structure includes said emission layer and has a ridge stripestructure for limiting an injection current path, and said ridge stripestructure is formed above a region excluding a middle portion of saidstriped mask.
 16. The nitride semiconductor light emitting deviceaccording to claim 1, wherein on said emission layer, a p-type blockinglayer containing Al, a p-type guide layer, and a p-type AlGaN claddinglayer are stacked in this order.
 17. A method of manufacturing a nitridesemiconductor light emitting device of claim 1, wherein a growthinterruption interval within a range from 1 to 180 seconds is providedbetween crystal growth of said well layer and crystal growth of saidbarrier layer over a substrate.
 18. The method of manufacturing anitride semiconductor light emitting device of claim 1, wherein aftercrystal-growing said emission layer on a substrate and furthercrystal-growing p-type layers thereon, substrate temperature isdecreased to a temperature within a range from 650° C. to 900° C. withsupply of ambient gas containing nitrogen and NH₃, followed byterminating supply of NH₃ and holding the substrate temperature withinthat range for a prescribed time period.